The Fukushima event is slowly building to become the most significant event
in the history of nuclear power. I will continue to update this page to cover all aspects of this event and also to
relate this event to US reactors. Please stop back when you can.

April 11 update: According to
MSNBC, the
Japanese government's nuclear safety agency raised the crisis level of the
Fukushima Dai-ichi power plant accident from 5 to 7,
the worst on the international scale and on par with the Chernobyl accident 25
years ago. Japan earthquake One month on, Japan rattled by big aftershock Somber
ceremonies mark a month since tsunami Japan to stop pumping radioactive water
into sea NYT: Nuclear disaster reveals Japan temps' plight Millions of yen
turned in, Japanese police say Aftershock in Japan unnerves traumatized nation
Time-lapse of aftershocks Images of chaos, destruction Kyodo news agency said
the government's Nuclear Safety Commission has estimated the amount of
radioactive material released from the reactors in Fukushima, northern Japan,
reached a maximum of 10,000 terabequerels per hour at one point for several
hours, which would classify the incident as a major accident according to
the INES scale.

What Happened

An
earthquake of magnitude 9 occurred at 2:46 PM on March 11, 2011 about 80
miles out in the Pacific Ocean east of Sendai, Japan. That earthquake resulted
in a tsunami over 40 feet high which hit the Fukushima Daiichi plant. Units 1, 2
and 3 were operating. Units 4, 5 and 6 were shutdown. All of Unit 4's reactor
fuel load had been placed in the Unit 4 spent fuel pool. Subsequent to the
original earthquake, numerous
additional quakes have occurred.

The plant's diesel generators started when the
loss of offsite power (LOOP)
occurred. About an hour later, the diesel generators became unavailable.
LOOP events are very infrequent but can have high safety significance. In
this case, the diesel generators could not be easily restored. Such an event is
considered a
Station Blackout. US plant procedures usually consider such an event would
likely last up to 8 hours. As a result, the decay heat in the recently shutdown
reactors 1, 2 and 3 and the Unit 4 spent fuel pool, which contained the recently
discharged full core offload of spent fuel, caused the cooling system
temperatures to rise. Eventually the temperatures rose to the point that
increased water evaporation or boiling occurred. Eventually the spent fuel was
uncovered and the fuel temperatures rose because water flow was not removing the
heat. Eventually, fission product gases passed through the zircaloy cladding.
Temperatures above 2200F are considered undesireable. Eventually, the steam
interacted with the zircaloy to produce hydrogen gas. Water/steam mixtures (with
gases) passed from the reactor through pressure relief valves to the torus (also
called suppression pool) which contains water. Fission product gases and
hydrogen built up in the drywell. In turn, the pressure buildup in the drywell
resulted in the operators determining the need to reduce pressure. When that was
done, hydrogen was released to the Reactor Building where a hydrogen explosion
occurred. An excellent seminar
presentation, Nuclear Crisis in Japan , provided by Dr. Alan Hanson on March 21 at the
Stanford
University's Freeman Spogli Institute for International Studies presented an analysis based on information
received to date.

Design Basis of Nuclear Plants

This nuclear plant was designed in the 1960s. While earthquakes and tsunamis
were considered in the design, events of this magnitude were not considered.

There are 6 reactors at the Fukushima Daiichi site: Unit 1 is a BWR-3, Units
2 through 5 are BWR-4s, and Unit 6 is a BWR-5.

BWR is the abbreviation for the Boiling Water Reactor. The BWR reactor typically allows bulk boiling of the water in
the reactor. The operating temperature of the reactor is approximately 570F
producing steam at a pressure of about 1000 pounds per square inch. Current BWR
reactors have electrical outputs of 570 to 1300 MWe and are about 33% efficient.

Reproduced by permission

In the figure above, water is circulated through the Reactor
Core picking up heat as the water moves past the fuel assemblies. The
water eventually is heated enough to convert to steam. Steam separators in the
upper part of the reactor remove water from the steam.

The steam then passes through the Main Steam Lines to the
Turbine-Generators. The steam typically goes first to a smaller
High Pressure (HP) Turbine, then passes to Moisture
Separators (not shown), then to the 2 or 3 larger Low Pressure
(LP) Turbines. In the sketch above there are 3 low pressure turbines,
as is common for 1000 MWe plant. The turbines are connected to each other and to
the Generator by a long shaft (not one piece).

The Generator produces the electricity, typically at about
20,000 volts AC. This electrical power is then distributed to a
Generator Transformer, which steps up the voltage to either
230,000 or 345,000 volts. Then the power is distributed to a switchyard or
substation where the power is then sent offsite.

The steam, after passing through the turbines, then condenses in the
Condenser, which is at a vacuum and is cooled by ocean, sea,
lake, or river water. The condensed steam then is pumped to Low Pressure
Feedwater Heaters (shown but not identified). The water then passes to
the Feedwater Pumps which in turn, pump the water to the
reactor and start the cycle all over again.

The BWR is unique in that the Control Rods, used to shutdown
the reactor and maintain an uniform power distribution across the reactor, are
inserted from the bottom by a high pressure hydraulically operated system. The
BWR also has a Torus (shown above) or a Suppression
Pool. The torus or suppression pool is used to remove heat released if
an event occurs in which large quantities of steam are released from the reactor
or the Reactor Recirculation System, used to circulate water
through the reactor.

In the US, radiation exposure due to this event should be of little concern.
Radiation levels are rarely above background. Radionuclide deposition is being
monitored by the Department of Energy and Environmental Protection Agencies, as
well as various state agencies, e.g. Washington, Oregon, and California. Data
from governmental agencies has been limited. One example of DOE data is provided
in a 2010 PowerPoint presentation (Original
DOE - Converted to HTML). However,
the University of Washington Physics department and University of California
Nuclear Engineering department have both been monitoring airborne and liquid
concentrations and posting them.

Appendix B, Annual Limits on Intake (ALIs) and Derived Air Concentrations
(DACs) of Radionuclides for Occupational Exposure; Effluent Concentrations;
Concentrations for Release to Sewerage, of Title 10 Code of Federal
Regulations Part 20 (10CFR20),
Standards for Protection against Radiation, identifies the
regulatory limits for all radionuclides for workers and the public.
Concentration limits in air and water for releases from nuclear power plants are
presented in
Table
2. As an example, click for
I-131 and
Cs-137 limits. To relate these limits to the data you may see in the news,
1E-10 microcuries per milliliter (mci/ml)
is the same as 100 picocuries per ml (pci/ml) or 3.7 Becquerel/ml or
3.7E-3 Becquerel/m3).

Converting Units

Reporting on radiation dose, radionuclide concentrations for the Japanese
reactor event has typically been expressed in the SI (metric) system of units.
Units of measurement are:

Radiation Absorbed Dose, SI unit is the Gray where 1 Gray is equal to the
absorbed dose of 100 Joules/kilogram (100 Rads)

Radiation Dose Equivalent, SI unit is the Sievert where 1 Sievert is the
absorbed dose in grays multiplied by the Quality Factor for the type of
radiation (QF = 1 for X-Rays, Gamma, or Beta Radiation; 20 for Alpha
particles, multiple-charged particles, fission fragments, and heavy particles
of unknown energy; 10 for neutrons of unknown energy and high-energy protons;
2 to 11 for monoenergetic neutrons per
Table 1004(b).2 of 10CFR20.

Radioactivity, SI unit is the Becquerel (Bq) where 1 Bq is equal to 1
disintegration per minute.

The US nuclear industry typically uses the original units not SI units.

Radiation Absorbed Dose unit is the Rad where 100 Rads are equal to 1
Gray.

Radiation Dose Equivalent unit is the Rem where 100 Rem are equal to 1
Sievert.

Radioactivity unit is the Curie where 1 Curie is equal to 3.7 E10
disintegrations per second or 3.7E10 Becquerels (Bq).